The consensus from email marketing experts and official documentation sources (Google, Yahoo, AMP.dev, and Mailjet) is that registration for sending AMP emails is a one-time process per sender address. Once a sender's email address is registered, and they meet the required criteria (including DKIM and sender reputation), they do not need to re-register or resubmit email templates for subsequent AMP email sends from that same address. Google, Yahoo, and Mail.Ru are often part of this joint registration process.